This invention relates to a positioning control apparatus for a synchronous motor.
A positioning control apparatus for a synchronous motor was disclosed in detail by a U.S. Pat. No. 4,151,449 entitled "Positioning Control Apparatus" (hereafter will be called the prior invention) owned by the same applicant of the present invention.
The positioning control apparatus disclosed by the prior invention belongs to an open-loop control system in which the control of a synchronous motor is performed in a way similar to that of a stepping motor.
As is well known, there are several demerits which are inherent to an open-loop positioning control system. For example, a momentary power failure may cause the evaporation of the memory in the register which stores the coded signal representing the instantaneous angular position .phi., and then, when the power is restored, the motor may follow to an angular position indicated by the new contents of the register which has no relation to the position .phi..
A closed-loop positioning control apparatus for a synchronous motor was also disclosed in detail by a U.S. Pat. No. 4,051,419, entitled "Control System of an Alternating-Current Motor" owned by the same applicant of this invention. But the control apparatus disclosed by the U.S. Pat. No. 4,051,419 is rather complicated and is not adapted to be used for a small-power synchronous motor. And the open-loop type control apparatus disclosed by the prior invention can be transformed to a closed-type positioning control apparatus when an error-detecting device is supplemented to the apparatus and the input pulse frequency is kept to be proportional to the amount of the error signal.
It is, therefore, a principal object of the present invention to provide a control apparatus whereby a closed-loop positioning control is attained by a simple attachment to the open-loop type control apparatus disclosed by the prior invention.
These and other objects of the present invention will become manifest upon a study of the following description and the accompanying drawings.